1. Field of the Invention
The invention concerns a method for the production of a spiral-shaped heating element, by winding an oblong base material onto a mandrel while forming a spiral with the base material and by equipping the spiral ends with contacts for electrical connection. Furthermore, the invention concerns a device for the production of a spiral-shaped heating element with a mandrel and with a device for feeding the oblong base material to the mandrel on whose casing surface the base material is wound in a spiral shape. Furthermore, the invention relates to a heating element for an infrared radiator, with the element being formed in a spiral shape with ends equipped with contacts for electrical connection. Additionally, the invention concerns an infrared radiator with a housing that encloses a spiral-shaped heating element. which is equipped with electrical connections.
2. Description of the Related Art
Infrared radiators are generally equipped with a heating coil, which consists of a metallic wire of high electric resistance. The heating coil is generated from plastic deformation of the metallic wire by winding it in a spiral shape onto a mandrel and subsequently removing the mandrel. The ends of the spiral produced this way are then equipped with metallic contact parts for electrical connection of the heating coil.
On prior art devices used for the production of such metallic heating coils, a mandrel is provided, which is fed the resistance wire continuously from a supply reel and which has a casing surface wound in a spiral shape. During the winding process, either the mandrel is moved in the direction of its longitudinal axis or the feeding mechanism of the wire is moved along the mandrel""s longitudinal axis.
A heating element and an infrared radiator of the kind described above are prior art devices from German Utility Model No. 90-03181. On the infrared radiator described there, a heating coil that is wound in a spiral shape onto a carrier tube is provided within a jacket tube, with the spiral coil being hooked up to connecting lines for electrical connection purposes.
In British Patent No. 2,233,150, an infrared radiator has the heating element made in the shape of a carbon strip that is arranged within a quartz glass tube, which is closed on both ends. The carbon strip consists of a number of graphite fibers that are arranged parallel to each other and have the shape of a strip. For the purpose of electrical connection, the carbon strip is equipped on both sides with metallic end caps. Generally, the front ends of the carbon strip are pinched into these end caps. The caps are connected with a spirally bent metallic wire, which in turn is connected with the electrical duct that reaches through the closed ends of the jacket tube.
A similar infrared radiator is described in German Patent No. 4,419,285. The heating element in this infrared radiator consists of a carbon strip that is arranged in a meandering shape. with the strip being formed by several connected partial sections with ends fastened on supports.
The carbon strip allows quick temperature changes so that prior art infrared carbon radiators excel through their high reaction speed. However, according to the Stefan-Boltzmann law, the radiation capacity of a radiating body decreases considerably with decreasing temperatures so that the radiation capacity of prior art carbon strips is low at comparatively low temperatures of the heating element, e.g. below 1000xc2x0 C.
In its original condition, the carbon strip consists of composites. A variety of fine carbon fibers is set mechanically within a thermoplastic embedding compound, such as resin. Only limited plastic deformation can be achieved on the carbon strip in this condition so that the prior art method and device for the production of a spiral-shaped heating element made of this material are not suitable.
The invention relates to a method and a device for the production of a spiral-shaped heating element made of material that contains carbon fibers. Furthermore, the invention relates to a heating element that, on the one hand, excels due to its low thermal inertia and, on the other hand, provides a high radiation capacity at comparatively low temperatures. The invention also relates to an infrared radiator that is produced by utilizing such a heating element.
As far as the manufacturing procedure for the heating element is concerned, the task is resolved by the invented method described above with a base material that comprises carbon fibers, which are encased into a thermoplastic embedding compound, by warming the base material to a temperature that softens the embedding compound, winding the softened base material onto the mandrel while forming a spiral, and setting the spiral shape by removing the embedding compound.
The invented method allows the production of spiral-shaped heating elements made of a base material that contains carbon fibers to be possible. Due to the spiral shape, the surface of the heating element produced in this way is considerably larger than the surface of a cylindrical, oblong heating element of the same length. The larger surface in turn leads to higher radiation capacity of the heating element at those temperatures.
The base material is initially available in an oblong shape, for example, as a thread or a strip. By warming the base material to a temperature at which the embedding compound softens, a state is reached that allows plastic deformation of the base material. The base material is shaped in the warmed state by winding it onto the mandrel in a spiral shape. The spiral shape created in this way is then set. This setting is achieved through complete or partial removal of the embedding compound, thus avoiding or reducing subsequent plastic deformation of the heating element during its intended usage in an infrared radiator. Upon complete or partial removal of the embedding compound, the spiral shape is still maintained. Removal can occur through chemical reactions, for example, through reaction with a solvent or by vaporization or pyrolysis (thermal decomposition).
A preferred version of the method includes removal of the embedding compound through annealing of the spiral to a temperature and in an atmosphere at which the embedding compound is converted into volatile matter. Conversion into volatile matter occurs through vaporization or decomposition of the embedding compound or through reactions with components of the surrounding atmosphere. Volatile components can be removed easily.
Heating of the spiral should preferably occur without oxygen, for example, in a closed reactor, inert gas, or in a vacuum. This way oxidation of the carbon fibers is avoided.
The base material can be warmed either across its entire length or in sections. It has been proven beneficial to warm the base material continuously in sections across its length, with the respectively softened section being wound onto the mandrel. Winding of the base material turns out to be especially simple if the mandrel rotates at the same time around its longitudinal axis.
The mandrel can also be warmed to a temperature that exceeds the softening temperature of the embedding compound, either across its entire length or in sections.
The preferred shape for the base material is a strip. A heating spiral that has been produced with strip-shaped base material distinguishes itself through a particularly large surface, and thus high radiation capacity.
In light of this fact, the utilization of strips proved particularly beneficial if the thickness was between 0.1 mm and 0.5 mm and the width was between 2 mm and 20 mm.
With regard to the fixture used to execute the method, the above-mentioned task is resolved by providing a heating device that affects the base material in the area of the casing surface of the mandrel for production of a spiral-shaped heating element made of a base material that comprises carbon fibers, which are encased into a thermoplastic embedding compound. This heating device can be adjusted to a temperature above the softening temperature of the embedding compound.
The base material is warmed to a temperature above the softening temperature for the embedding compound through a heating device. By allowing the heating device to affect the base material in the area of the casing surface of the mandrel, the base material is softened in the respective sections that are fed to the mandrel in such a way that the base material reaches a state of plastic deformation and can be wound in a spiral shape onto the casing surface of the mandrel. Transfer of the heat from the heating device onto the base material can occur through contacts, radiation, current, or convection. The heating element can affect the base material directly or indirectly through interposition of a device for transmission. It is important only that the heating device affects the base material in the area of the casing surface of the mandrel.
A device whose design allows the mandrel to rotate around its longitudinal axis and on which the heating device can be moved relative to the mandrel proved to be beneficial. In order to be able to wind the base material in a spiral shape onto the rotating mandrel, either the mandrel itself is moved in the direction of its longitudinal axis or the base material is guided continuously along the casing surface of the mandrel via the feeding device. In the first case, the heating and feeding devices can be fixed locally; in the latter, the heating and feeding devices can be moved along the longitudinal axis of the mandrel. With the ability to move the mandrel and the heating device relative to each other, a specific and locally limited warming process of the base material can be achieved.
The warming process of the base material is particularly simple and exact when using a version of the invented device where the heating device can be moved with the help of a linear guiding mechanism that runs parallel to the longitudinal axis of the mandrel.
It is beneficial to equip the feeding device with a first driving mechanism, with which it can be moved in a direction that is parallel to the longitudinal axis of the mandrel, with a second driving mechanism, which is coupled electrically or mechanically to the first driving mechanism, being provided to be able to move the heating device. By coupling the two driving mechanisms together, the movements of the heating device and the feeding device can be synchronized, thus enabling exact local warming of the base material.
A heating device that includes a hot air fan proved particularly advantageous.
With regard to the spiral-shaped heating element, the above task is resolved by the fact that the element consists of a certain layout of carbon fibers that are connected with each other.
At the same length, the surface of the spiral-shaped heating element is considerably larger than the surface of the prior art, oblong, strip-shaped heating element. At a given temperature, the larger surface leads to comparatively higher radiation capacity. The invented heating element therefore excels also due to its lower thermal inertia while simultaneously offering high radiation capacity, which becomes noticeable especially at comparatively low temperatures.
In this respect, it is particularly beneficial if the heating element is formed as a spiral-shaped carbon strip. The spiral shape of the heating element allows it to increase its surface by up to three times that of the prior art, oblong, strip-shaped carbon strip.
Should the base material used for production of the invented heating element be a composite that comprises a number of fine carbon fibers, which are fixed mechanically in a thermoplastic embedding compound, such as a resin, then the base material is preferably brought into a spiral shape and set in accordance with the above-described procedure.
The invented infrared radiator includes a housing that encloses a spiral-shaped heating element, which is equipped with electrical connections, as described above. The spiral-shaped heating element is produced from a base material that contains carbon fibers. Such an infrared radiator distinguishes itself from others due to a high radiation capacity, especially in wavelength ranges of 1.5 xcexcm to 4.5 xcexcm.
The following description provides more detailed explanations of the invention with the help of an example of one version.